Dairy Chemistry And Biochemistry

254 DAIRY CHEMISTRY AND BIOCHEMISTRY

Table 5.6 Distribution of milk salts

~~

Soluble Colloidal (%)

Concentration

Species (mg1-l) YO form

Sodium

Potassium

Chloride

Sulphate

Phosphate

Citrate

Calcium

Magnesium

500

1450

1200

100

750

1750

1200

130

92

92

100

100

43

94

34

67

Completely ionized 8

Completely ionized 8

Completely ionized -

Completely ionized -

10% bound to Ca and Mg - 57

51% H,PO-

39% HPOf-

85% boud to Ca and Mg

14% Citrate3-

1% HCitrate2-

35% Ca2+ 66

55% bound to citrate

10% bound to phosphate

Probably similar to calcium 33

0 Citrates: 14% tertiary (Citrate3-), 1 % secondary (HCitrate2-) and 85%

0 Phosphates: 51% primary (H2P0,), 39% secondary (HPO:-) and 10%

bound to calcium and magnesium.

bound to calcium and magnesium.

Combining this information with the distribution of the various salts
between the colloidal and soluble phases (Table 5.5), gives the quantitative
distribution of the salts in milk shown in Table 5.6.
It should be possible to determine experimentally the concentrations of
anions such as HPOi- and Citrate3- in milk using ion-exchange resins or
by nuclear magnetic resonance spectroscopy, but no such experimental
work has been reported and available data are by calculation only.
Making certain assumptions and approximations as to the state of
various ionic species in milk, Lyster (1981) and Holt, Dalgleish and Jenness
(1981) developed computer programs that permit calculation of the concen-
trations of various ions and soluble complexes in typical milk diffusate. The
outcome of both sets of calculations are in fairly good agreement and are
also in good agreement with the experimentally determined values for those
species for which data are available. The values calculated by Holt, Dal-
gleish and Jenness (1981) are shown in Table 5.7.
The ionic strength of milk is around 0.08 M.

5.7.3

Ca2+ and Mg2+, along with H', play especially important roles in the
stability of the caseinate system and its behaviour during milk processing,
especially in the coagulation of milk by rennet, heat and ethanol. The

Measurement of calcium and magnesium ions